Currently, research and development of digital coherent optical receivers are being promoted, and it is expected to achieve a significant reduction in the device cost of light communication systems. In a digital coherent optical receiver, a signal is decoded by mixing a received signal light and a local oscillation light.
On the other hand, in a conventional heterodyne optical receiver, automatic frequency control (AFC) is executed so that a center frequency of an intermediate-frequency (IF) signal obtained by mixing a signal light and a local oscillation light is kept constant. Therefore, an automatic starting method to bring a signal band of the IF signal into the capture range of an AFC is proposed.
In this automatic starting method, the wavelength of local oscillation light is swept by increasing or decreasing an electric current or a temperature applied to a laser diode (LD) used as a local oscillation light source.
FIG. 18 illustrates the relationship between the variation in oscillation wavelength and the bias current in a three-electrode distributed-feedback LD. The variation in oscillation wavelength against the change in the current IC supplied to the center electrode is greater than the variation in oscillation wavelength against the change in the current IS supplied to the side electrode by approximately one order of magnitude, thereby the wavelength of local oscillation light can be swept by controlling the current IC.
FIG. 19 is a flowchart of the automatic starting method in a heterodyne optical receiver including such an LD. First, the optical receiver performs a self-diagnostic operation, and at the same time, sets initial values of the current IC and the temperature (step 11). Subsequently, monitoring of the IF signal output is performed (step 12), and it is determined whether or not the IF signal output is present (step 13).
If the IF signal output is not present (No in step 13), whether or not the current IC is at its minimum is checked (step 14), and if the current IC is not at the minimum, then the current IC is decreased by a predetermined value (step 15), and thereafter, controls in step 12 and after are repeated.
On the other hand, if the current IC is at the minimum (Yes in step 14), the current IC is left at the minimum, and at the same time the temperature is set to its maximum (step 16). Subsequently, monitoring of the IF signal output is performed (step 17), and it is determined whether or not the IF signal output is present (step 18). If the IF signal output is not present (No in step 18), it is checked whether or not the temperature is at its minimum (step 19). If the temperature is not at the minimum (No in step 19), then the temperature is decreased by a predetermined value (step 20), and thereafter, controls in step 17 and after are repeated.
On the other hand, if the temperature is at the minimum in step 19 (Yes in step 19), the current IC and the temperature are set to their maximums (step 16), and thereafter, controls in step 17 and after are repeated.
If the IF signal output is present in step 13 or 18 (Yes in step 13 or 18), it is determined whether the frequency of the signal light or the frequency of the local oscillation light is higher on the basis of whether the IF signal is on the real side or on the image side (step 21).
If the frequency of signal light is higher than the frequency of local oscillation light, AFC is turned on without change (step 22), the IF signal output and the frequency discriminator (FD) output are checked (step 23), and then control is terminated. On the other hand, if the frequency of signal light is lower than the frequency of local oscillation light in step 21, controls in step 22 and after are performed after the frequency of local oscillation light is modified. Thereby the IF signal can be brought into the capture range of AFC.
FIG. 20 illustrates the detectable range of the IF signal. Since the IF signal passes through a band pass filter, the IF signal can be detected only when its frequency is within the pass band 31 of the band pass filter. Therefore, the IF signal 33 within the pass band 31 will be detected, while a signal having a signal band outside of the pass band 31, such as the IF signal 32 or 34, will not be detected in for example the initial state of the automatic starting method.
Patent Document 1: Japanese Laid-open Patent Publication No. 05-48540